Medical electrodes comprising a sheet or film of a hydrophilic gel as a conductive member interfacing with the skin of a patient are well known in the art. Hydrogel sheets adapted for use in medical electrode-related applications are commercially produced by, among others in the United States, Promeon, a Division of Medtronic, Inc., (Brooklyn Center, Minn.); Valleylabs, Inc., a Division of Pfizer (Boulder, Colo.); Biostim, Inc. (Princeton, N.J.); Lectec Corp. (Eden Prairie, Minn.); and Conmed (Utica, N.M.).
Numerous U.S. patents disclose hydrophilic gels and medical electrodes employing them. The following illustrate the state of that prior art.
U.S. Pat. No. 3,357,930 (electrically conductive transparent films comprising a polymeric matrix in gel form, a plasticizer therefor, an ionized electrolyte soluble in the plasticizer, and an ionizable solvent, e.g., solid polyvinyl alcohol, glycerine, lithium chloride and silica).
U.S. Pat. No. 3,911,906 (electrode with skin-interfacing film of a pressure sensitive adhesive containing fine electrically conductive particles, e.g., an acrylic copolymer containing carbon powder).
U.S. Pat. No. 3,993,049 (electrode comprising a pliant patch of a formaminated material covered on the side adapted to be placed on the skin with a salt-containing adhesive).
U.S. Pat. No. 3,994,302 (stimulating electrode in which the skin contacting element is an ion-exchange material, e.g., a vinyl pyridine grafted to a polyethylene base).
U.S. Pat. No. 3,998,215 claims an electrically conductive pad which employs a hydrogel impregnated with a fibrous carrier. The polymers disclosed therein as operable require a chemical cross-linking agent. The commercial version thereof sold by the patentee (Minnesota Mining and Manufacturing Co.) has poor skin adhesion and contains bubbles (the latter presumably due to the viscosity of the starting gel and/or the technique employed to impregnate the fibrous carrier with the starting polymer solution). Bubbles in the conductive pad are undesirable because they create local areas of altered electrical properties.
Since the issuance of U.S. Pat. No. 3,998,215, numerous other patents employing a hydrophilic gel as an electrically conducting means which interfaces with the skin of the patient have been issued. The following are illustrative of such patents.
U.S. Pat. No. 4,008,721 (tape electrode comprising a skin-contacting layer of adhesive material, e.g., acrylic copolymer).
U.S. Pat. No. 4,054,714 (electrically conductive adhesive useful for binding together surfaces of electronic devices, comprising a polymeric binder, conductive particles whose surfaces are a noble metal and a normally liquid polyhydric alcohol).
U.S. Pat. No. 4,067,342 (tape electrode for transmission of electrical signals into the body through the skin employing a tape having a surface of a conductive material combined with an adhesive, e.g., acrylic polymer adhesive, and a second surface with the conductive material comprising a magnetic substance).
U.S. Pat. No. 4,094,822 (electrode having a cup, which is taped to the skin, containing a semi-solid adhesive polymeric material, e.g., a mixture of polyvinyl alcohol, boric acid, CMC, glycerol and water and an electrolyte, e.g., AgCl or a zinc salt).
U.S. Pat. No. 4,066,078 (electrode with a skin-interfacing film having adhesive, plastic and hydrophilic properties, e.g., produced from an interpolymer comprising (a) 10-90 parts of an ester of an .alpha.,.beta.-olefinically unsaturated carboxylic acid and a mono- or polyhydric alcohol; (b) 90-10 parts of an .alpha.,.beta.-olefinically unsaturated comonomer; and (c) at least 0.02 parts of a crosslinking agent comprising a difunctional monomer).
U.S. Pat. No. 4,092,985 (disposable electrode comprising an elastically stretchable layer of water-permeable porous webbing permeated with a high water content liquid or semisolid conductive medium).
U.S. Pat. No. 4,109,648 (electrode assembly comprising a self-supporting body of hydrogel, e.g., hydroxyethyl methacrylate polymerized with ammonium persulfate and sodium metabisulfite around graphite fiber).
U.S. Pat. No. 4,125,110; U.S. Pat. No. Re. 31,454 (electrode comprising as a skin-interfacing substrate, a colloidal dispersion of a naturally occurring hydrophilic polysaccharide such as karaya, and a salt in an alcohol as a continuous phase).
U.S. Pat. No. 4,141,366 (electrode for transmitting electrical signals through the skin employing a normally "dry" adhesive which is activated at the time of application by a suitable solvent).
U.S. Pat. No. 4,273,135 (an essentially dry electrode employing as the conductive interface a cohesive, conformable, nonionic hydrophilic synthetic polymer plasticized with a monomer, e.g., glycerol. The electrode is applied to abraded skin wet with normal saline solution or water). This patent contains a detailed description of prior art electrodes.
U.S. Pat. No. 4,274,420 (an electrode similar to that of U.S. Pat. No. 4,125,110 in which the adhesive substrate comprises a karaya gum matrix supporting an electrically conductive fluid).
U.S. Pat. No. 4,300,575 (an electrode with a conductive element composed of karaya, carbon black, isopropyl alcohol and karaya gum conductive solution).
U.S. Pat. Nos. 4,317,278; 4,318,746 and 4,362,165 (electrodes comprising an annulus of foam with an electrode gel in the central region of the annulus, which gel is the subject of U.S. Pat. No. 4,318,746 and is composed of two polymers, one of which is hot water soluble, e.g., kappa carrageenan, and the other is not, e.g., hydroxypropylmethylcellulose, and which contains a potassium salt to enhance the gel's conductivity).
U.S. Pat. Nos. 4,365,634; 4,393,584; and 4,522,211 (electrodes with adhesive layer secured to a semi-flexible plastic-like sheet, and formed from a known electrically conductive adhesive, e.g., Johnson & Johnson Co.'s "Bioadhesive", disclosed in U.S. Pat. No. 4,066,078, or in U.S. Pat. Nos. 4,008,721; 3,998,215; 3,993,049; and 3,911,906; preferably a hydrophilic material disclosed in U.S. Pat. Nos. 3,822,238; 4,156,066 and 4,156,067).
U.S. Pat. No. 4,383,529 (iontophoretic electrode device with a semi-solid hydrophilic hydrated gel formed, e.g., from agar, a protein or a synthetic polymer, e.g., methyl cellulose).
U.S. Pat. No. 4,458,696 (TENS electrode with an extensible interfacing layer of up to 10 mils thickness comprised of a carrier portion coated with an electrically conductive adhesive, preferably a 75:25 butyl acrylate-acrylic acid copolymer neutralized with methyl diethanolamine to which are added a water-soluble plasticizer and tackifier, as described in U.S. Pat. No. 3,065,770).
U.S. Pat. No. 4,515,162 (electrode pad comprising a tacky crosslinked hydrogel adhered to an electrode terminal plate, e.g., a polyacrylic acid and a polyacrylic acid salt, water, and a compound containing at least two epoxy groups, as cross-linking component, and optionally a tackifier, e.g., glycerine, propylene glycol or polyethylene glycol, an electrolyte material, e.g., sodium chloride or potassium chloride, a pH controlling agent, a flexibility imparting agent, an antifungal agent, and the like).
U.S. Pat. No. 4,524,087 (electrode with a conductive adhesive thereon which is a swellable, dermally-nonirritating, conformable, coadhesive, ionic hydrophilic polymer, e.g., produced by UV polymerizing a mixture consisting of triethylene-glycol-bis-methacrylate dissolved in acrylate acid to which is added glycerol and potassium hydroxide in water, using a free radical initiator to initiate polymerization, e.g., a photoinitiator).
U.S. Pat. No. 4,543,958 (electrodes with conductive adhesive film comprising a naturally occurring karaya gum, e.g., available in sheet form from Lectec Corp. or as described in U.S. Pat. Nos. 3,357,930; 3,993,049; 4,066,078; and 4,141,366).
European Published Patent Application 83 305 770.6 (Publication No. 0107376) discloses poly(vinyl pyrrolidone) gel dressings which are non-rigid, sterile, tacky, transparent and absorbent, which have been crosslinked by ionization radiation and which are useful in the treatment of wounds, skin disorders and burns. These gel dressings are formed from 10% to 25%, preferably 15-20% and most preferably 20%, crosslinked poly(vinyl pyrrolidone) and water and irradiated with 1-3 Mrads radiation. Other patents also describe hydrophilic polymers crosslinked into gels, e.g., U.S. Pat. No. 3,998,215 which has poly(vinyl alcohol) as the relevant polymer in concentrations up to 30%.
Furthermore, hydrophilic gels derived from crosslinked poly(ethylene oxide) polymers are described in U.S. Pat. Nos. 3,264,202; 3,419,006; 3,898,143; 3,993,551; 3,993,552; 3,993553 and 3,900,378. These references do not disclose hydrogels with the unique characteristics of the present invention.
U.S. Pat. Nos. 4,750,482 and 4,699,146, both issued to Sieverding, describe irradiating poly(vinyl pyrrolidone) formulations to yield hydrophilic elastomeric adhesives. The conductive formulations contain low molecular weight (300-600 MW) polyethylene glycols as plasticizers for the adhesives and require high doses of irradiation to achieve preferred results. The characteristics of the present invention are not disclosed.
U.S. Pat. Nos. 4,684,558; 4,706,680 and 4,777,954, all issued to Keusch et al., describe tacky adhesive poly(ethylene oxide) gels which may be formulated to also be conductive. The hydrophilic poly(ethylene oxide) solutions are crosslinked by irradiation. These references also disclose the utility of poly(vinyl pyrrolidone) as the crosslinked polymer and include an example of PVP (MW=360,000) in a conductive formulation. Like the Sieverding references, cited above, these patents do not disclose or teach the desirable characteristics possible in the present invention.
U.S. Pat. No. 4,593,053, issued to Jevne et al., describes hydrophilic gel compositions including those comprised of PVP and poly(vinyl alcohol). Chemical crosslinking agents are used, however. Likewise, U.S. Pat. No. 4,192,827, issued to Mueller et al., describes hydrophilic gels which utilize relatively exotic comonomers or copolymers which are not crosslinked by irradiation.
U.S. Pat. Nos. 4,539,996 and 4,554,924, both issued to Engel, disclose conductive adhesive electrodes which utilize adhesive precursors comprised of water-soluble polyhydric alcohols and additional components which are crosslinked via a chemically initiated free-radical process. These patents give no guidance as to the methods of formulation of a cross-linked absorbent flexible adhesive poly(ethylene oxide) (PEO) gel.
Not only does each type of polymer behave differently, even with PEO itself, certain formulations irradiated with a given dose were generally regarded as being non-tacky and non-adhesive. The usefulness of these non-tacky, non-adhesive materials in adhesive applications was believed to be quite limited. Indeed, very few water-soluble crosslinkable polymers can meet all of the requirements of strength, absorbency, flexibility, adhesiveness, and slow drying characteristics possessed by the PEO gels of this invention. Past teachings and uses of crosslinked PEO hydrogel sheets emphasized either the smoothness and ease of removal of the sheet from a surface or the highly tacky adhesive stringy compositions described by Keusch et al. Although the materials used in this earlier work included PEO, the highly desirable properties and characteristics of the PEO hydrogels of the present invention were not recognized.
U.S. Pat. No. 4,989,607, issued to Keusch, discloses a conductive hydrogel comprised of a cohesive uniform mixture of poly(vinyl pyrrolidone), a viscosity-enhancing hydrophilic polymer, and an effective amount of electrolyte.
U.S. Pat. No. 5,143,071, issued to Keusch, discloses adhesive hydrophilic gels comprised of a homogeneous uniform mixture of water and at least one water-soluble high molecular weight polymer. The mixtures may be derived from poly(ethylene oxide) and water or poly(vinyl pyrrolidone), a viscosity-enhancing hydrophilic polymer and water.
The previous teachings on dose versus polymer concentration for PEO in water may be summarized as follows: (1) the dose to achieve a crosslinked PEO hydrogel is inversely proportional to concentration (U.S. Pat. No. 3,419,006); (2) the dose should be greater than 0.52 Mrads (U.S. Pat. No. 3,264,202, claim 6); and (3) at certain doses a tacky adhesive hydrogel can be obtained from a formulation having a given polymer concentration. However, at doses which exceed the levels of (3), above, these same formulations were believed to provide non-tacky, non-adhesive PEO hydrogels of limited utility (U.S. Pat. No. 4,684,558).
Each of these criteria would not be helpful in synthesizing a highly conductive, slow drying PEO gel and would, in fact, be misleading. Poly(ethylene oxide) water systems also have a unique response to high energy radiation. At low concentrations, crosslinking occurs by indirect effects, i.e., initiated with the solvent, whereas at high concentrations (the limit being PEO in solid form) the poly(ethylene oxide) does not crosslink at all. This behavior makes it even more difficult to predict the conditions to achieve effective combinations of absorbency, strength, flexibility, and adhesiveness. Also, the addition of humectants in significant quantities was not addressed and would significantly change the conditions of synthesis.
Although there exist many examples of cohesive and/or conductive hydrophilic gels, none embodies the unique combination of properties disclosed herein. In particular, previous gels were non-tacky and non-adhesive. Yet others may have had tacky, aggressively adhesive, cohesive, or sticky characteristics useful in self-adhesive applications. Yet other formulations yield such a high degree of adhesiveness (i.e., are aggressively adhesive) that skin or hair may be damaged upon removal of the gel sheet. Some hydrogels have a tendency to dry during prolonged use and therefore adversely affect conductivity stability of electrodes incorporating those hydrogels. Consequently, a self-adhesive hydrogel which is more comfortable to use and one which may be formulated to be sufficiently conductive without drying out prematurely during use would be of significant utility.
Hence, none of the currently available hydrogels meet all the criteria of the slow drying adhesive hydrophilic gels of this invention, viz., which are formed from a polymeric material which is dermatologically inert, i.e., one which contains no organic solvents, residual monomer, chemical cross-linking agents or substantial quantities of uncrosslinked polymer, but which contains a humectant; which is a viscoelastic solid, i.e., readily conforms to non-flat areas of the skin; which is sufficiently adhesive to adhere firmly to the skin, so that there is little likelihood of its falling off during use, yet is not so adhesive that it causes pain and/or damage to the skin upon removal; which is adequately adhesive to moist as well as to dry skin and to soiled as well as to clean skin, so that skin pre-preparation with organic solvent or detergent or abrasive is not required; which substantially maintains its water content during prolonged use, thereby making possible electrodes incorporating hydrogels that maintain substantially constant conductivity and do not dry out during prolonged use; which has a good shelf life in a sealed container which does not transmit water vapor through its walls; and whose properties do not readily deteriorate between the time the container bearing the adhesive sheet is sealed and the time at which the container is opened. The adhesive sheets of this invention possess all of these and many other advantageous properties.